Not sure if this is the appropriate place for this question as its fairly unique. Figured there is vast knowledge here in the building/construction section and will get an answer or pointed in the right direction.

Have a raised enclosed porch that I have opened up as a dining area into the main house. Current insulation that is under the floor is suspect and I want to open/remove the bottom and insulate appropriately (spray in foam or foam-board with a radiant foil barrier). As for heating - I have added 2 duct runs and have installed some under floor heat wire as a just in case.

My question is when sealing it back up - can i use regular cement board or should i use a different product? I see there are fibercement panels but the cost, when compared to standard cement backer board, is expensive. The board will be an exterior install but will not see direct sunlight or water. Currently there is plywood under there - not sure how old it is - but it looks like its ready to be replaced. Not sure what to use but want something to last and also look decent.